other options to source rights free images

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Search Creative Commons is a really useful, convenient search portal for locating all manner of content which you can use freely.
Photos For Class, Pics4Learning and PhotoPin have some copyright free images and IconArchive the Open Clip Art library and The Kidz Page have clipart.
Pixabay has both images and clipart.

Dan Leeman in a edublogger post shares The Ultimate Directory of Free Image Sources. Buffer Feed has a great post listing 53+ Free Image Sources For Your Blog and Social Media Posts which you might like to check through.
Dustin Senos also has an interesting list of Stock Photos That Don't Suck.
The Atlantic has A Guide to the Web's Growing Set of Free Image Collections made up of the museums and research institutions that have large, high-quality, free-to-use collection of historically or aesthetically notable images online, (note you need to wait for the ad to time out).
You should also look at More information about using images from Flickr and more